1. Field of the Invention
The present invention is generally concerned with torsional damper devices comprising two coaxial parts mounted to rotate relative to one another within a defined range of relative angular movement against circumferentially acting elastic means.
2. Description of the Prior Art
As is known, this type of torsional damper device is normally incorporated in the design of a friction clutch, particularly for automotive vehicles, for example, in which case one coaxial part carries a friction disk designed to rotate with a first shaft, in practice a driving shaft and the motor output shaft in the case of an automotive vehicle, constituting a part referred to hereinafter for convenience as the driving part, whereas the other of said coaxial parts is carried on a hub designed to rotate with a second shaft, in practice a driven shaft and the gearbox input shaft in the case of an automotive vehicle, constituting a part referred to hereinafter for convenience as the driven part.
This type of torsional damper device is used to permit regulated transmission of rotational torque applied to one of its coaxial parts where the other is itself subject to a rotational torque, in other words, to filter vibrations which may arise at any point in the kinematic system in which it is incorporated, extending from the motor to the axles in the case of an automotive vehicle.
The present invention is more particularly directed towards the case in which the elastic means used in such a torsional damper device, disposed between the coaxial parts constituting same, comprise a plurality of helical springs extending in a substantially tangential direction relative to a circumference of the damper assembly and disposed between said coaxial parts in the circumferential direction, each of said springs being housed partially in an opening formed for this purpose in the driving part and partially in an opening also formed for this purpose in the driven part.
In practice, it is common practice to divide the springs used into at least two separate groups and to delay the action of at least one of these groups during relative angular movement between the driving and driven parts, in order to modulate the stiffness of the elastic means operative between the driving and driven parts according to the extent of relative angular movement between them and thus to improve the adaptation of the assembly to the specific filtering effect required.
To this end, whereas, in the rest configuration of the damper, all springs used are usually disposed without circumferential clearance in the corresponding openings in the driving part, those whose action is to be delayed are, in this rest configuration, disposed with circumferential clearance in the corresponding openings in the driven part, so that they are operative only from the time at which, by virtue of the relative angular movement between the driving and driven parts, this circumferential clearance is absorbed.
As a corollary to this, at the start of such relative angular movement, the only springs which act are those which, in the rest configuration of the damper, are disposed without circumferential clearance in the corresponding openings in the driven part and in the corresponding openings in the driving part.
In practice, these springs are of relatively less stiffness than those acting jointly with them subsequently.
More often than not, these springs of relatively low stiffness, which together form a first or low-stiffness damper stage, the other springs also used together forming a second or high-stiffness damper stage, are disposed in openings in the driving part and in the driven part separate from those in which said other springs are disposed.
This results in an increase in the number of openings required in the driving and driven parts, the disadvantage of which is two-fold.
On the one hand, for a given final stiffness, the degree of mechanical weakening of the driving and driven parts due to the openings formed in them increases with the number of openings and may become excessive, whereas, on the other hand, with a given and acceptable number of openings in the driving and driven parts, the resulting final stiffness may be insufficient for the torque to be transmitted between said parts.
To overcome this problem, it has already been proposed, notably in French Pat. No. 1 537 961 dated July 21, 1967, to dispose within the relatively stiffer springs the relatively less stiff springs intended to act first, so that the latter benefit from the openings formed in the driving and driven parts for the relatively stiffer springs, by having them project in the circumferential direction beyond the latter so that they come into action before them.
Although this arrangement has given and can still give satisfaction, it leads in practice to structures which are relatively complex and expensive.
In particular, in a case such as the present one where one of the coaxial parts comprises a disk and the other comprises two guide rings parallel thereto, in which the openings for the springs are formed, it is necessary, in order to counterbalance the effect of centrifugal force on the springs of relatively less stiffness and to ensure their correct retention in position, to reinforce the guide rings by means of disks and backing disks, and to extend the openings in these in the circumferential direction by means of a notch at one circumferential end, for insertion of the corresponding end of the springs of relatively lower stiffness.
The result is an increase in the overall axial dimension of the damper, and mechanical weakening of the parts in which said notches are formed. This also applies to the disk when no reinforcing disk or backing disk is used, these notches then being formed in the disk.
In all cases, by their very nature, the springs of relatively lower stiffness employed in this way do not permit other than linear change in the torque transmitted between the coaxial parts as relative angular movement between them progresses.
A general objective of the present invention is to provide an arrangement which can overcome these disadvantages and which, by not requiring the use of complex retaining means, is relatively simple and economic, while ensuring effective filtering of vibration and opening up a new field of application of the torsional damper devices concerned.